RFID reading apparatus serve for the identification of objects and products and are used inter alia to automate logistical movements. RFID transponders fastened to the products are read out at an identification point, above all on a change of the owner of the product or on a change of the transport means, and information is optionally written back into the transponder. The detected information is used to control the forwarding and sorting of goods and products. Important applications for automatic identification are logistical distribution centers, for instance of package shippers, or the baggage check-in at airports.
The RFID reading apparatus excites RFID transponders located in its reading range by electromagnetic radiation via its antenna to emit the stored information, receives the corresponding transponder signals and evaluates them. For this purpose, the UHF (ultra-high frequency) range is frequently used since there is an established framework here in the standard ISO 18000-6 and in addition transponders at different distances from some millimeters up to several meters can be read out. UHF RFID transponders are available in very compact construction designs and can also accordingly be attached to very small objects. RFID reading apparatus in the UHF range are, however, not usable for a number of applications, for example in a milling machine for recognizing the milling tools used, in the currently available construction designs due to the housing size.
There is therefore a need for particularly compact designs of the antennas that take up as little construction space as possible within the RFID reading apparatus. The required antenna size in this respect, however, generally depends on the frequency. This is in turn not a free parameter since, for example, a frequency range between 850 MHz and 950 MHz is provided for UHF, within which range frequencies can be used which are determined for specific countries. An antenna for an RFID reading apparatus can therefore not easily adopt a base surface of less than λ/4×λ/4, where λ is the wavelength of the RFID signal.
It is admittedly conceivable in some applications to work with very small external antennas; however, this means an additional installation effort. In addition, known antennas for the RFID range that allow a smaller housing size than λ/4 at all are linearly polarized or have such a small antenna gain that transponders can only be read in direct contact or at best up to a distance of some few centimeters. However, circularly polarized antennas or at least elliptically polarized antennas are preferably used in RFID reading apparatus to supply transponders with energy in all angular positions and to read them out. A further demand on the antenna is robustness with respect to interference in the environment such as metal surfaces next to or behind the RFID reading apparatus or additional dielectrics directly in front of the antenna. There are no antennas that combine these criteria within themselves.
So-called inverted-F antennas are known in the prior art that are installed in a number of compact and portable devices. These antennas have their name due to their F shape. The bars are formed from the infeed line and from an additional lateral short-circuit connection to the ground plane, the vertical line is the actual antenna arm. A frequently particularly flat and compact construction shape is the so-called PIFA antenna (PIFA, planar inverted F-antenna) that can be directly integrated on a circuit board as a patch antenna in a microstrip technique. A disadvantage of these antennas in RFID applications is, however, their linear polarization.
US 2009/0179816 A1 discloses an antenna system having two PIFAs that are arranged at a right angle to one another in a plan and at their bases. One of the PIFAs is controlled with a phase offset of 90° to generate waves that are circularly polarized overall. No good circular polarization is achieved with the oppositely disposed alignment that is with the fact that both PIFAs face away from the common base and with the use of only two PIFAs. In addition, there are no measures to direct the radiation of the antenna in a preferred direction.
CN 203 895 604 U combines a total of eight inverted-F antennas in two subgroups of four respective antennas via a feed network for a circularly polarized antenna. The antenna is rather large in construction, not solely due to the eight antennas, but because the antenna structure is not flat in itself and the feed network is located on a different plane than the antenna structure. Such a three-dimensional structure with additional geometrical and circuit-board planes additionally means a high effort and thus additional costs, particularly for mass production.
CN 103 606 756 A shows a further circularly polarized antenna composed of four inverted-F antennas that, however, has comparable disadvantages due to its layer construction and due to the antennas being oriented in the layer direction.
Four antennas each having an L-shaped slit are arranged in a plane and are connected to a feed network that is not located at the same plane in US 2012/0299789 A1.
Different construction shapes are presented in US 2011/0018774 A1 by which a circulation polarization is created with a plurality of monopole antennas and a corresponding feed network. The antennas extend obliquely over a ground plane. A free space internally between the antennas can be used by further elements.